The present invention relates to a method commonly used for measuring flow speed which can be categorized into manometric (pressure detecting), thermal, sonic, and rotation measurement. The manometric method utilizes a pitot tube or various head meters. The method is excellent in that the devices involved are simple and invulnerable. Furthermore, it requires no calibration in principle, since the method is based on the fundamental laws of fluid mechanics. Thus, the pitot tube has been the standard instrument for the flow speed measurement. However, the pitot tube method requires complicated manipulation or instrumentation when used for the flow of varying direction, a well-known Dynes instrument or its modern versions being a good example. Head meters such as a venturi tube and an orifice meter can be only applicable to the unidirectional, confined flow in a conduit.
The thermal method such as hot-wire and thermometer technique is based on the dependence of cooling rate on the flow speed. Response of a hot-wire or thermocouple anemometer is fast thereby making the method useful for turbulence measurements. However, this method is not used in operational measurements, because deposit of dust, rain or snow can cause very large errors.
The sonic or acoustic method which relies on the dependence of the transit time of a sonic pulse on the transmitting medium is also very useful in the study of high-frequency flow measurements. However, this method is not used in operational measurements either for the similar reasons to the thermal method. Both hardware and data processing involved are complicated and the system is very expensive.
Thus, the method using the rotation meters is the only method that is employed in long-term routine measurements in the field. Cup or propeller anemometers and current meters are accurate and durable, and, in a usual environment will give years of reliable measurements with little maintenance. However, these instruments too are subject to erroneous operation where heavy snowfall and freezing prevent rotors from rotating. Also, rotors are occasionally blown out by strong gusts of heavy storms.